Conventionally, for a liquid crystal display device, a backlight unit that emits light from a backside of a liquid crystal panel (LCD) to illuminate the liquid crystal panel has been used. The backlight unit is normally configured by using a light source for illumination, a light guide plate for diffusing light emitted from this light source to irradiate the liquid crystal panel with the light, and components such as a prism sheet and a diffusion sheet for uniformizing the light emitted from the light guide plate.
Recently, this backlight unit has been used not only for the liquid crystal display device in the above-mentioned manner but also for a planar lighting device which serves for indoor or outdoor illumination or illuminates a liquid crystal display panel, an advertisement panel, an advertisement tower or a billboard from the backside.
Regarding not only the planar lighting device but also the recent liquid crystal display device, there has been a demand for thinning and lower power consumption. To realize this, light guide plates of various shapes have been proposed (refer to Patent Document 1, Patent Document 2, and Patent Document 3).
Patent Document 1 discloses a light guide plate including a groove having a substantially rectangular section on a side opposite to a light exit plane, for arranging a bar-shaped light source, and a diffusion system formed on the plane opposite to the light exit plane and the groove to diffuse light from the light source.
Patent Document 2 discloses parallely-used light guide plates each configured by including a transparent substrate which has a pair of protruded edges jointed together to freely contact each other on opposing entrance end planes so that light exit planes can be wider than reflection planes on the opposing entrance end plane sides, thereby forming a fitting groove for fitting a linear light source between the entrance end planes in a bottom plane, light guiding means formed in the reflection plane of the transparent substrate, for modulating a density by setting a width-direction intermediate position between the opposing entrance end planes as a peak, and transmission adjusting means formed in the protruded edges of the light exit plane or into strip shapes in a light exit plane in-plane direction of the protruded edges, for reducing and adjusting transmission of light of light sources.
Patent Document 3 discloses a light guide plate including a top surface which is a flat plane and a bottom surface which is an inclined plane including a curve where a sectional shape is thinner as becoming farther from a light source, in which a relationship of θf1≦θn1 is satisfied, θn1 and θf1 respectively denoting inclined angles of the light guide plate bottom surface at a position relatively near the light source and a position far from the light source, and an inclined angle of the bottom surface at a thinnest position is 0°.
For the liquid crystal display device, a backlight unit (planar lighting device) that emits light from a backside of a liquid crystal display panel to illuminate the liquid crystal display panel has been used. The backlight unit is configured by using a light guide plate for diffusing light emitted from a light source for illumination to illuminate the liquid crystal display panel, and components such as a prism sheet and a diffusion sheet for uniformizing the light emitted from the light guide plate.
Currently, large liquid crystal televisions predominantly use a so-called direct type method which includes a light guide plate disposed immediately above the illumination light source. In this method, a plurality of cold cathode tubes serving as a light source are provided on a back surface of the liquid crystal display panel, and the inside of the backlight unit serves as white reflection surfaces to secure uniform light-amount distribution and necessary brightness.
In order to achieve an even light amount distribution with the backlight unit of the direct type, however, the liquid crystal display panel needs to have a thickness of about 30 mm in a direction perpendicular to the liquid crystal panel. The backlight unit is expected to become thinner in the future, but it is thought that, in the direct type, the realization of a backlight unit having a thickness of 10 mm or less will be difficult in terms of irregular light-amount.
A backlight unit that couples light emitted from a light source to a light guide plate and emits light from a top surface of the light guide plate to the outside has been proposed.
For example, Patent Document 4 describes a planar light source device configured by including a light source, a light guide plate having a single refractive index, and a reflection sheet disposed in a backside of the light guide plate. The light source is installed in a light-source-insertion conical concave portion disposed in the backside of the light guide plate. In the backside of the light guide plate, light scattering dots for scattering light in the light guide plate are disposed. At least a part of light from the light guide plate is reflected on a plane and/or the backside of the light guide plate, and then emitted from the entire plane of the light guide plate.
Patent Document 4 describes, as light guide plates used for the planar lighting device thus configured, a flat light guide plate and a light guide plate whose thickness becomes thinner from a center to a peripheral portion.
In such a planar light source device, the light source is installed in a light-source-insertion conical concave portion disposed in the backside of the light guide plate. Thus, it is described that the planar light source device can be thinned.
In such a planar light source device, light emitted forward from the light source enters the light guide plate while being refracted toward a side direction by the side surface of the light source insertion concave portion. Thus, it is described that the amount of light emitted forward from the light source into the light guide plate can be reduced, and brightness unevenness caused by high brightness of the light guide plate in front of the light source is prevented to enable realization of a uniform brightness distribution in the light exit plane of the light guide plate.
Patent Document 1: JP 3074747 B
Patent Document 2: JP 2001-42327 A
Patent Document 3: JP 2002-75036 A
Patent Document 4: JP 3427636 B